1888.] 



Report on Hygrometric Methods. 



335 



perature of the surrounding air. Each experiment involved upwards 

 of thirty readings of weight, pressure, and temperature. The' tem- 

 perature readings were corrected by means of a special comparison at 

 Kew. Of the eighty-two observations thirty-two are retained as being 

 free from any known disturbing causes, and from them it appears 

 that, with d equal to 0622, the pressure deduced by the chemical 

 method is on the average greater by 03 mm. than that given in 

 Regnault's table of vacuum pressures, as recalculated in Landolt and 

 Bornstein's tables. This difference is very small compared with the 

 discrepancies from Dalton's Law observed by Regnault in the. case of 

 water vapour. 



(iv.) With regard to the fourth question ; if the observations be 

 employed to determine the value which must be substituted for d, the 

 specific gravity of saturated steam referred to air at the same tem- 

 perature and pressure, the mean value of d so obtained is 0'6245, 

 which agrees very closely with 0'6240, the mean value for the same 

 range of temperature deduced from Clausius's calculations based on 

 thermodynamical reasoning. The value 0'622 is probably correct if the 

 air is not nearly saturated ; in that case the measure of the pressure of 

 vapour in the air is 2/622 greater than it would be if the same air were 

 reduced in temperature (at constant pressure), until it was saturated. 



(v.) The one observation of the second series with saturated air 

 gives a result 0*18 mm. smaller than the tabulated pressure, and thus 

 with the twelve experiments of the first series confirms the results of 

 Regnault's observations. To account for this, it is suggested that 

 air which is very nearly or quite saturated, would deposit some of its 

 moisture on the glass tubes used to conduct it from one vessel to 

 another. This behaviour of nearly saturated air has been already 

 noticed, and it is confirmed by the observations on dew-point in- 

 struments, and moreover, by experiments directly intended for the 

 purpose, quoted in a note. 



Details are given of observations with Regnault's hygrometer and 

 Dines's hygrometer when exposed in glass vessels between the satu- 

 rater and the drying tube. The two instruments are separately dis- 

 cussed. With Regnault's instrument, after some practice, two different 

 observers obtained practically identical results. In ordinary observa- 

 tions, the observed temperatures of the dew-point were below the 

 temperature of saturation, but seldom by more than 1° 0. A con- 

 siderable amount of uncertainty was shown to be attached to the 

 readings, and by very close inspection readings of the dew-point were 

 obtained above the temperature of saturation, in one case by as much 

 as 0-7° C. 



From the experiments with Dines's hygrometer, it appears that the 

 instrument is likely to give very easy determinations of the dew-point 

 that are within small limits of error ; but that if the instrument be 



